The present invention deals with a process for the preparation of polyurethane (henceforth abbreviated PU) rigid foams from conventional starting components in the presence of cyclopentane or mixtures of cyclopentane and/or cyclohexane and at least one compound homogeneously miscible with cyclopentane and/or cyclohexane having a boiling point below 35.degree. C. selected from the group consisting of alkanes, cycloalkanes having a maximum of 4 carbon atoms, dialkylethers, cycloalkylene ethers and fluoroalkanes, as well as additionally water as a blowing agent. The present invention also deals with using the PU rigid foams for foaming the hollow areas in low temperature housing compartments or in heating elements as well as using these PU rigid foams as insulating material for composite elements.
2. Description of the Related Art
The preparation of composite or sandwich elements made from a PU rigid foam and at least one top layer of a rigid or elastic material such as, for example, paper, plastic films, metal sheets, glass non-wovens, backing panels, etc. is well known. Also known is foaming the hollow spaces in household appliances such as, for example, low temperature housing compartments, that is, refrigerators or freezer chests or hot water tanks using PU rigid foam as a heat insulating material. In order to avoid cavities the foamable PU reaction mixture must be injected into the hollow space which is to be insulated within a short time. To foam such household appliances, typically low pressure machines, but preferably high pressure machines are used.
Typical insulating PU rigid foams can be prepared conventionally by reacting organic polyisocyanates with one or more higher molecular weight compounds having at least two reactive hydrogen atoms, preferably polyester polyols and/or polyether polyols as well as typically while using lower molecular weight chain extending agents and/or crosslinking agents in the presence of blowing agents, catalysts and optionally auxiliaries and/or additives. By properly selecting the starting components one can obtain PU rigid foam having a very low coefficient of thermal conductivity and good mechanical properties.
A comprehensive overview concerning the preparation of PU rigid foams and their use as covering layers or preferably core layers in the composite elements as well as their use as insulation layers in low temperature or heating technology has been published for example in Polyurethanes, Plastics Handbook, Vol. 7, 1st Ed. 1966, edited by Dr. R. Vieweg and Dr. A. Hoechtlen and in the 2nd Edition of 1983 edited by Dr. Gunther Voertel, Carl-Hanser Verlag, Munich and Vienna.
Chlorofluoroalkanes, preferably trichlorofluoromethane, are used worldwide on a large scale as blowing agents in the preparation of insulating PU rigid foams. A disadvantage of these propellant gases is an environmental burden since they are suspected of contributing to the degradation of the ozone layer in the stratosphere.
Along with the aforesaid trichlorofluoromethane, other physically effective blowing agents are also used in the preparation of PU rigid foams. One example is found in DC-C-1 045 644 (U.S. Pat. No. 3,391,093) which discloses gaseous hydrocarbons having not more than 3 carbon atoms such as methane, ethane, ethylene, propane and propylene and halogenated hydrocarbons such as, for example, chloromethane, dichlorodifluoromethane, dichlorofluoromethane, chlorodifluoromethane, chloroethane and dichlorotetrafluoroethane as well as, octafluorocyclobutane, and hexafluorocyclobutane and hexafluoropropane. Another example is found in Belgium patent 596,608 which discloses halogen alkanes, such as for example, 1,1-difluoro-2,2-dichloroethane, 1,2-difluoro-1,2-dichloroethane, 1,1-dichloroethane, 1-fluoro-1,2-dichloroethane, 1-fluoro-2,2-dichloroethane, 1,2-dichloroethane, trichloroethane, tetrachloroethane, 1-fluoro-1,2,2-trichloroethane, 1-bromoethane, and 1,1,2-trifluoro-2-chloroethane. Another example is found in PCT Application WO 89/00594 which discloses 1,1,1-trichloroethane which is used when mixed with other blowing agents.
The aforesaid blowing agents have somewhat of a disadvantage in that they are toxic, or compared to trichlorofluoromethane they possess a lower gas yield when blowing PU foam because of their boiling point, or they make the PU rigid foam have a lower insulating effect, and/or they cause the foam to shrink. Additionally they lead to the formation of voids in the foams core or to the partial collapse of the foam even during the foaming process itself.
Especially the saturated and unsaturated hydrocarbons, specifically n-pentane, suitable for foaming polystyrene, possess thermal conductivities which are too high to generate PU rigid foams having the required insulating properties. Thus, for example, the thermal conductivity of n-pentane is 150.multidot.10.sup.-4 W/m.multidot..degree.K. and that of n-butane at 25.degree. C. is even 163.multidot.10.sup.-4 W/m.multidot..degree.K.
Another blowing agent is carbon dioxide which according to GD-A 21 16 574 can be dissolved under pressure in at least one starting component for the preparation of PU rigid foam; said carbon dioxide can be thermally cleaved from salts such as, for example, carbamates; carbonates such as, for example, ammonium carbonate, or from bicarbonates, or can be formed from the reaction of isocyanate with water to form urea groups. Along with the established industrial processing difficulties when using solid carbon dioxide or gaseous carbon dioxide under pressure, this method of preparing PU rigid foams has a significant disadvantage in that the carbon dioxide, due its very high diffusion rate, diffuses very quickly through the matrix of the PU foam. In addition, at 25.degree. C. carbon dioxide has a thermal conductivity of 164.multidot.10.sup.-4 W/m.multidot..degree.K.; this value lies at the level of that of n-butane and is 85% poorer than the formally used trichlorofluoromethane.